Educate the scrolling tree about visual and layout viewports. This is runtime-switchable,
so we push the enable flag to via the root state node, then push the layout viewport,
and the min/max scroll position that contstrain it, through frame state nodes.

When a scroll happens, we compute a new layout viewport when the visual viewport hits
an edge, and push that down through setScrollLayerPosition() since it's used to position
fixed and sticky layers.

When the main thread gets notified about an async scroll, we set the new layout viewport
on the FrameView, but do so in such a way that does not trigger layout. This is OK because
we do a RenderLayer update which udpates all the layoutViewport-dependent state, and is
necessary to avoid repaints every main thread update.

Educate the scrolling tree about visual and layout viewports. This is runtime-switchable,
so we push the enable flag to via the root state node, then push the layout viewport,
and the min/max scroll position that contstrain it, through frame state nodes.

When a scroll happens, we compute a new layout viewport when the visual viewport hits
an edge, and push that down through setScrollLayerPosition() since it's used to position
fixed and sticky layers.